Optical wavelength division multiplexed system using wavelength splitters

ABSTRACT

An optical wavelength division multiplexed system uses wavelength splitters to split channels included in input light into different paths within the system. Odd-numbered channels are split into one path, and even-numbered channels are split into another path, providing increased isolation between channels. Using filters, the system then drops one or more of the isolated, split channels into paths referred to as dropped paths and allows the remaining channels to continue through the filters into output paths. The dropped paths are then combined into one, common dropped path, and the output paths are also combined into one, common output path.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation patent application that claims the benefit ofU.S. patent application Ser. No. 10/390,784, filed Mar. 19, 2003, whichis a divisional application that claims the benefit of U.S. patentapplication Ser. No. 09/185,505, filed Nov. 4, 1998, now U.S. Pat. No.6,556,742.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical systems, and, moreparticularly, to optical wavelength division multiplexed systems inwhich channels are multiplexed and demultiplexed.

2. Description of the Related Art

An optical wavelength division multiplexed (WDM) system is an opticalsystem carrying many different wavelengths, or frequencies, of light.The frequencies are closely spaced, and, from an information systemsperspective, are also referred to as channels, which channels carryinformation.

In WDM systems as described above, optical filters, such as opticalwavelength demultiplexers, have a very important role. One configurationof the channel dropping systems is shown in FIG. 1 where channels havingpredetermined frequencies are dropped by a filter 10 from the mainsignal stream of light A into another light path C (referred to as thedropped channel(s)), and all other channels are transmitted along path B(referred to as the transmitted channels). In this system, thefrequencies of the dropped channels are predetermined. If channels atarbitrary frequencies can be dropped, more flexible systems will bebuilt. One of the interesting filters is a wavelength tunable filter, anexample of which is shown in FIG. 2. As shown in FIG. 2, wavelengthtunable filter 12 drops (or separates out) arbitrary frequencies fromthe main signal stream of light A to another light path C (referred toas the dropped channel), and transmits the remaining frequencies alongpath B (referred to as the transmitted channels). An interesting tunablefilter is an acousto-optic waveguide filter, which filters frequenciesof light from the main stream in response to electric power provided tothe acousto-optical waveguide filter. The frequencies of light which aredropped from the main stream are determined by acoustic frequencieswhich are applied to the device, and, accordingly, may be dynamicallyaltered. If more than one acoustic frequency is applied simultaneouslyto the wavelength tunable filter, all of the corresponding channels aredropped.

With an acousto-optic waveguide filter, shown in FIG. 2, light wave A,which includes 4 frequencies (or channels) 1, 2, 3, and 4, is input towavelength tunable filter 12. Wavelength tunable filter 12 drops channelnumber 3 into path C, while transmitting remaining channels 1, 2, and 4along output path B. If the wavelength tunable filter 12 of FIG. 2 were,for example, an acousto-optic tunable filter, then the channel droppedalong path C would be in response to an output of an acoustic wavegenerator 14 provided in the filter.

As channels in WDM systems become more closely spaced, demultiplexershave increasing difficultly isolating one or more channels from theother input channels and extracting the selected channel(s). Because thechannels are so closely spaced in prior art WDM systems, problems withfiltering shape and crosstalk (or leaked light) affect the WDM systems,as do problems with instability of optical power in adjacent channels.Most troublesome in WDM systems of the prior art are problems ofcrosstalk and instability of optical power in adjacent channels.Problems of crosstalk and instability of optical power most seriouslyaffect the transmitted light, and problems of crosstalk most seriouslyaffect the dropped light, as explained with reference to FIGS. 3A, 3B,3C, 3D, 3E, 4, and 5.

Problems of filtering shape and of crosstalk in the context of filteringand dropping channels are explained with reference to FIGS. 3A–3E, whichrespectively show spectrums of light for input channels from whichchannel 3 is dropped.

FIG. 3A shows an ideal filtering shape surrounding channel 3. An idealfiltering shape F is rectangular, having a flattened top and a base ofequal width W with the top of the filtering shape (as shown in FIG. 3B).

However, because of imperfections in conventional filters, an idealfiltering shape is difficult to achieve. Three common problems whichoccur with conventional filters include providing a filtering shape Fwith a top that is not flat (FIG. 3C), a filtering shape F with a base(W) considerably wider than the top of the filtering shape F (FIG. 3D),and a filtering shape F filtering an input light wave shifted by adistance S from the center of the spectrum (FIG. 3E).

The example of the filtering shape shown in FIG. 3C results in a reducedamount of power present in channel 3, and, further, an altered shape ofthe optical spectrum in channel 3, after filtering. As shown in FIG. 3C,area C has been cut off of channel 3 by the filter F. To compensate forthis loss of power, the width of the filter F may be expanded, resultingin crosstalk from adjacent channels 2 and 4, as shown in FIG. 3D. Ofcourse, the shape of the filter F shown in FIG. 3D could have alsoresulted simply from imperfections in filter F which has a wider basethan the top width. Crosstalk C in channel 3 resulting from either ofadjacent channels 2 or 4 (crosstalk C is shown in FIG. 3E resulting fromadjacent channel 2) also occurs when the center of the spectrum of theinput light for channel 3 is shifted a distance S from the center of thespectrum of the filter F. Because of the close proximity of adjacentchannels 2 and 4 to channel 3, channel 3 receives crosstalk C upon beingfiltered.

FIG. 4 shows a spectrum of light transmitted along path B of FIG. 2. If,in the example shown in FIG. 2, channel number 3 is dropped to path C,the output stream B should include only remaining channel numbers 1, 2,and 4. However, as shown in the spectrum of transmitted light of FIG. 4,a small portion of the light in channel number 3 remains and istransmitted along path B. This transmission of a small portion of thelight in channel number 3 along path B, even though channel number 3 wasdropped into path C, is due to insufficient isolation of channel 3 fromchannels 1, 2, and 4.

Also as shown in FIG. 4, the optical powers in channels 2 and 4, whichare adjacent to channel 3, fluctuate in time and, therefore, providechannels 2 and 4 with instability in their signals. The problem ofinstability of the optical power due to the dropped channel (such aschannel 3) is most acute in the channels adjacent (such as channels 2and 4) to the dropped channel, but the instability also affectsnon-adjacent channels (such as channel 1), with decreasing intensity asthe distance (in terms of frequency) from the dropped channel increases.

FIG. 5 shows a spectrum of the dropped light transmitted along path C ofFIG. 2. As shown in FIG. 5, a problem arises with the dropped channel 3transmitted along path C because the light dropped into path C includeslight power contributions not only from channel 3 (which are desired)but, also, light power contributions from channels other than channel 3(such as channels 1, 2, and 4). Light power from channels 1, 2, and 4 isundesired and is leaked along path C. The problem of the leaked opticalpower into the dropped channel (such as channel 3) is most acute in thechannels adjacent (such as channels 2 and 4) to the dropped channel, butthe leak also occurs in non-adjacent channels (such as channel 1), withdecreasing intensity as the distance (in terms of frequency) from thedropped channel increases.

All of the above-mentioned problems are caused by interference betweenthe channels. As shown in FIGS. 4 and 5, and as explained above, theeffects on and of channel number 1 are weaker, because the channelposition of channel 1 is further from the position of channel 3. Atypical isolation of an acousto-optic waveguide filter is ˜20 dB for theadjacent channels when the channel spacing is 0.8 nm.

Therefore, filters providing increased isolation between the channels ofthe input light are desired. Also desired is a WDM system havingincreased isolation between the channels of the input light, but beingof low cost and providing low optical loss.

SUMMARY OF THE INVENTION

An object of the present invention is to increase isolation betweenchannels of input light.

Another object of the present invention is to drop a channel from inputlight and transmit output light free of crosstalk.

A further object of the present invention is to drop a channel frominput light and provide output light free of instability of opticalpower channels adjacent to a dropped channel.

Yet another object of the present invention is to drop a channel frominput light and output the dropped channel free of crosstalk.

To achieve the above-mentioned objects, the present invention is anoptical wavelength division multiplexed system using wavelengthsplitters.

The fundamental idea of the present invention is to split the evenchannels and the odd channels of the wavelength division multiplexedsystem into different optical paths, and then insert an optical filterinto each optical path.

The optical wavelength division multiplexed system of the presentinvention receives input light, and splits, using a splitter,odd-numbered channels into one path and even-numbered channels intoanother, different path.

After the input light (which includes alternating odd- and even-numberedchannels adjacent to each other) is split in the present invention, theresulting split light traveling along each path includes only channelswhich are not directly adjacent to each other, thereby increasing theisolation between the channels.

Then, in the present invention, filters (preferably tunable) placed ineach path drop the selected channel(s) from the input, split light, andboth the output transmitted channels and the dropped channels are outputby the present invention free of crosstalk present in the WDM systems ofthe prior art. In addition, the output transmitted channels are outputby the present invention free of the instability of optical powerpresent in WDM systems of the prior art.

In the present invention, an optical wavelength division multiplexedsystem comprises a splitter splitting input light into paths, one of thepaths including even-numbered channels and another of the pathsincluding odd-numbered channels, each of the paths comprising an opticalfilter. In another aspect of the present invention, an opticalwavelength division multiplexed system comprises a splitter splittinginput light into paths in which channels with channel numbers ofmultiples of 3, channel numbers of multiples of 3 plus 1 (includingchannel 1), and channel numbers of multiples of 3 plus 2 (includingchannel 2), are split into different paths, and each of the pathsincludes an optical filter.

In a further aspect of the present invention, an optical wavelengthdivision multiplexed system comprises splitters splitting input lightinto paths, in which channels with channel numbers of multiples of 4,channel numbers of multiples of 4 plus 1 (including channel 1), channelnumbers of multiples of 4 plus 2 (including channel 2), and channelnumbers of multiples of 4 plus 3 (including channel 3), are split intothe different paths, and each of the paths includes an optical filter.

In addition, the present invention includes a method of droppingchannels carried in input light received by an optical wavelengthdivision multiplexed system. The method of the present inventioncomprises splitting by the system a portion of the channels into onepath within the system and another portion of the channels into anotherpath within the system, such that neither of the paths carry adjacentchannels, and dropping by the system one of the channels into a droppedpath.

Further, the present invention includes a method of dropping channelscarried in input light received by an optical wavelength divisionmultiplexed system. The method of the present invention comprisessplitting by the system the channels into at least two, separate pathscarrying non-adjacent channels within the system, then dropping by thesystem one of the channels into a dropped path.

In addition, the present invention includes a method of isolating anddropping channels carried by input light. The method of the presentinvention comprises splitting by a splitter the channels into separatepaths, one of the paths carrying odd-numbered channels and another ofthe paths carrying even-numbered channels, then dropping by a filter oneof the channels into a dropped path.

These together with other objects and advantages which will besubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a channel dropping filter of the prior art.

FIG. 2 shows a wavelength tunable filter of the prior art.

FIG. 3A shows an ideal filtering shape.

FIG. 3B shows an ideal filtering shape in further detail.

FIGS. 3C, 3D, and 3E show three common problems which occur withconventional filters.

FIG. 4 shows a spectrum of transmitted light in the prior art.

FIG. 5 shows a spectrum of dropped light in the prior art.

FIG. 6 shows the principles of an optical wavelength divisionmultiplexed system of the present invention.

FIG. 7A shows a spectrum of an ideal filter.

FIGS. 7B–7D each show spectrums of light of a channel being filtered,respectively, by conventional filters.

FIG. 8 shows an optical wavelength division multiplexed (WDM) system 24of the present invention.

FIG. 9 illustrates an optical wavelength division multiplexed system(WDM) 30 with splitter of the present invention.

FIG. 10 shows an optical wavelength division multiplexed system (WDM) 40of the present invention with splitters and cascaded filters.

FIG. 11 shows an optical wavelength division multiplexed system 42 with4 paths, each with cascaded filters.

FIG. 12 shows a Mach-Zehnder interferometer with asymmetric arm lengthsof the prior art.

FIG. 13 shows a waveguide device of the prior art.

FIG. 14 shows a typical spectrum in one of two outputs of a Mach-Zehnderinterferometer.

FIG. 15 shows a desirable spectrum.

FIG. 16A shows an ideal splitter 66 of the present invention,

FIGS. 16B and 16C show spectrums of light in the output paths of thesplitter shown in FIG. 16A.

FIG. 17 shows another aspect of the present invention.

FIG. 18 shows phase added by phase elements 72 and 74 of the presentinvention.

FIGS. 19, 20, and 21 show implementations of phase elements 72 and 74.

FIG. 22 is a flowchart of the methods of the optical wavelength divisionmultiplexed systems of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles of the present invention are shown in FIG. 6, which showsoptical wavelength division multiplexed system (WDM) 16 of the presentinvention. In the optical wavelength division multiplexed system 16 ofthe present invention, input light A including channels 1, 2, 3, 4, . .. is received by splitter 18, which splits (or separates) the inputlight A into two optical paths A₁ and A₂. Splitters, which split inputlight based upon frequencies or wavelengths, are known in the art, andeither a conventional splitter, or, preferably, splitters as describedherein below, may be used as splitter 18 to split the input light A intochannels.

Also as shown in FIG. 6, in the WDM 16 of the present invention, oddchannels 1, 3, 5, 7, . . . travel along optical path A₁ and entertunable filter 20. Likewise, even channels 2, 4, 6, 8, . . . travelalong optical path A₂ and enter tunable filter 22. Although the use oftunable filters is preferable, filters 20 and 22 need not be tunable.Since the channel spacing of the light traveling along either opticalpath A₁ or A₂ is doubled as a result of having been split, the isolationbetween the adjacent channels in each optical path is much higher thanin the prior art.

The odd-numbered channels which are dropped are then output by filter 20along path C₁, and the even-numbered channels which are dropped areoutput by filter 22 along path C₂. Likewise, the remaining (or output)odd-numbered channels are output by filter 20 along path B₁, while theremaining even-numbered channels are output by filter 22 along path B₂.

Using the example put forth with reference to FIGS. 1–5, if channel 3 isthe only channel dropped by the WDM 16 of the present invention, thenchannel 3 is output along path C₁, while remaining channels 1, 5, 7, . .. are output along path B₁ and remaining channels 2, 4, 6, . . . areoutput along path B₂. In this example, no channels are output along pathC₂.

Since the channel spacing in WDM 16 of the present invention is twice aslarge as that in the prior art WDM, there are no channels adjacent tothe dropped channel(s), and, accordingly the crosstalk between channelsaffecting the dropped channel and the remaining channels, and theinstability of the optical power in the remaining channels areessentially eliminated. In the example discussed, dropped channel 3would receive a small amount of crosstalk from remaining channels 1 and5, and remaining channels 1 and 5 would receive a small amount ofcrosstalk from dropped channel 3. However, since the distance betweendropped channel 3 and each of remaining channels 1 and 5 is a distanceof 2 channel spacings, the amount of crosstalk between channels 1 and 3and between channels 3 and 5 would be small.

As shown in FIGS. 7A, 7B, 7C, and 7D, the problems of crosstalk andinstability of optical power of the prior art WDM systems mentioned withreference to FIGS. 3C–3E are solved by the WDM 16 of the presentinvention. FIG. 7A shows a spectrum of ideal filter F, being rectangularin shape and having width W consistent between the top and the base offilter F. FIGS. 7B–7D each show spectrums of light of channel 3 onlybeing filtered, respectively, by conventional filters F1, F2, and F3.The shapes of filters F1, F2, and F3 correspond, respectively, to theshapes of the filters shown in FIGS. 3C, 3D, and 3E, and would introducethe problems described in FIGS. 3C, 3D, and 3E into either the droppedchannel or the remaining channels or both but for the WDM 16 of thepresent invention, as explained.

Since the WDM 16 of the present invention, unlike the WDM of the priorart, splits the input channels A into odd-numbered channels A₁ andeven-numbered channels A₂, the next adjacent channel to each channel inchannels A, and A₂ is a distance of 2 channels away (in terms of theinput channels A).

The principles of the present invention shown in FIGS. 7B–7D areexplained with reference to the odd-numbered channels traveling alongpath A₁, but are also applicable to the even-numbered channels travelingalong path A₂ in the WDM 16 of the present invention.

In the examples shown in FIGS. 7B–7D, channel 3 is being dropped intopath C, by filter 20 of the WDM 16 of the present invention shown inFIG. 6, and channels 1, 5, 7, . . . remain to continue as outputtransmission channels along path B₁. Because the channel spacing in theWDM 16 of the present invention is twice the channel spacing in the WDMof the prior art, the size of the spectrum of the filter F1corresponding to channel 3 can be larger in the WDM 16 of the presentinvention than that used in the WDM of the prior art without introducingcrosstalk between channels. In FIGS. 7B–7D, X indicates that no opticalpower is provided at that location. Accordingly, the top T of the filterF1 is relatively flatter for a filter F shown in FIG. 3C. Therefore, areduction in the power of channel 3, after being dropped, would notoccur as readily in the WDM 16 of the present invention as in the WDM ofthe prior art.

Likewise, as shown in FIG. 7C, the width W of the base of the filter F2used in the WDM 16 of the present invention could be wider than thewidth W of the base of the filter used in the WDM of the prior artwithout introducing crosstalk or instability of optical power intoadjacent channels because the channel spacing between the channels beingfiltered in the WDM 16 of the present invention is twice as large as thechannel spacing between the channels being filtered in the WDM of theprior art.

In addition, as shown in FIG. 7D, since the channel spacing between thechannels being filtered in the WDM 16 of the present invention is twiceas large as the channel spacing between the channels being filtered inthe WDM of the prior art, filter F3 used in the WDM 16 of the presentinvention can be broader than the filter used in the WDM of the priorart without introducing crosstalk or instability of optical power intoadjacent channels. Therefore, even if the center of the spectrum forchannel 3 is shifted by a distance S from the center of the spectrum forfilter F3, filter F3 still covers the spectrum for channel 3 in the WDM16 of the present invention.

For the WDM 16 of the present invention shown in FIG. 6 to achieve thesame function as shown in FIG. 2, the output light B₁ and B₂ fromfilters 20 and 22, respectively, must be combined using splitter 26,and, likewise, the dropped light C₁ and C₂ from filters 20 and 22,respectively, must be combined using splitter 28 as shown in the WDM 24of the present invention in FIG. 8.

FIG. 8 shows an optical wavelength division multiplexed (WDM) system 24of the present invention which achieves the same function as the priorart apparatus shown in FIG. 2, but, further, includes the advantages ofthe WDM 16 of the present invention shown in FIG. 6. In the WDM system24 of the present invention, each filter 20, 22 receives only everyother channel. The WDM 24 of the present invention includes a splitter18 (as described herein above with reference to FIG. 6) and filters 20and 22 (also as described herein above with reference to FIG. 6).

More particularly in the WDM 24 of the present invention, filter 20would receive from path A₁ channels 1, 3, 5, 7, . . . . Likewise, filter22 would receive from path A₂ channels 2, 4, 6, 8 . . . . Therefore, inthe WDM 24 of the present invention shown in FIG. 8, the next adjacentchannel to any particular channel, after the input light A has beensplit by splitter 18, is effectively a distance of two channels awayfrom the particular channel.

Thereafter, channels from path A₁ (which would include odd-numberedchannels only) dropped by filter 20 travel along path C₁, and channelsfrom path A₁ (which would also include odd-numbered channels only)remaining (or not dropped) by filter 20 travel along path B₁. Likewise,channels from path A₂ (which would include even-numbered channels only)dropped by filter 22 travel along path C₂, and channels from path A₂(which would also include even-numbered channels only) remaining (or notdropped) by filter 22 travel along path B₂.

Then, the WDM 24 of the present invention recombines the channelstraveling along paths C₁ and C₂ by splitter 28 into dropped channelstraveling along path C. Further, the WDM 24 of the present inventionrecombines the channels traveling along paths B₁ and B₂ by splitter 26into remaining, transmitted output channels traveling along path B.

The following examples further explain the above-mentioned concepts ofthe WDM 24 of the present invention shown in FIG. 8. If input light Aincludes channels 1, 2, 3, 4, 5, 6, 7, 8, . . . , then splitter 18splits the input light A into channels 1, 3, 5, 7, . . . traveling alongpath A₁, and channels 2, 4, 6, 8, . . . traveling along path A₂.

If only channel 3 is to be dropped, then filter 20 drops channel 3 intopath C₁, and allows remaining channels 1, 5, 7, . . . to continue alongpath B₁ to splitter 26. Filter 22 does not drop any of channels 2, 4, 6,8, . . . into path C₂, but allows all channels 2, 4, 6, 8, . . . tocontinue along path B₂ to splitter 26. When splitter 26 recombines thelight traveling along paths B₁ and B₂, the resultant output light Bincludes channels 1, 2, 4, 5, 6, 7, 8, . . . When splitter 28 recombinesthe light traveling along paths C₁ and C₂, the resultant dropped light Cincludes only channel 3.

On the other hand, if both channels 3 and 6 are to be dropped, thenfilter 20 drops channel 3 into path C₁, and allows remaining channels 1,5, 7, . . . to continue along path B₁ to splitter 26. Filter 22 dropschannel 6 into path C₂, and allows channels 2, 4, 8, . . . to continuealong path B₂ to splitter 26. When splitter 26 recombines the lighttraveling along paths B₁ and B₂, the resultant output light B includeschannels 1, 2, 4, 5, 7, 8, . . . . When splitter 28 recombines the lighttraveling along paths C₁ and C₂, the resultant dropped light C includeschannels 3 and 6.

The above-mentioned scheme of isolating channels in WDM 24 of thepresent invention can be extended. In a WDM of the present invention,there are multiple ways in which the channels could be divided. One waywould be to have an optical wavelength division multiplexed systemincluding a splitter splitting input light into paths, in which thechannels with the channel numbers of multiple of 3, the channel numbersof multiple of 3 plus 1 (including channel 1), and the channel numbersof multiple of 3 plus 2 (including path 2), are split into differentpaths, and each path includes an optical filter.

Another way of dividing the channels in the optical wavelength divisionmultiplexed system of the present invention includes splitters splittinginput light into paths, in which channels with channel numbers ofmultiples of 4, channel numbers of multiples of 4 plus 1 (includingchannel 1), channel numbers of multiples of 4 plus 2 (including channel2), and channel numbers of multiples of 4 plus 3 (including channel 3),are split into the different paths, and each of the paths includes anoptical filter.

FIG. 9 illustrates an extended scheme of the present invention in whichan optical wavelength division multiplexed system (WDM) 30 with splitterof the present invention includes multiple splitters which splitchannels included in input light A multiple times into multiple paths.Each path includes a filter.

In the WDM 30 of FIG. 9, the channel numbers which are multiples of 4plus 1 travel to a first arm (or path) A₁₁, the channel numbers whichare multiples of 4 plus 3 travel to a second arm (or path) A₁₂, thechannel numbers which are multiples of 4 plus 2 travel to a third arm(or path) A₂₁, and the channel numbers which are multiples of 4 travelto a fourth arm (or path) A₂₂. With this scheme, the effective channelspacing in each arm is four times larger than in prior art systems. Thisfurther increases the isolation between channels over the prior artsystems.

As in the WDM 24 of FIG. 8, in the WDM 30 of FIG. 9, splitter 18 splitsinput light A into two different paths, with alternate channelsallocated to the different paths. Since there are two layers ofsplitters in the WDM 30, the input light A (including channels 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 11, 12, . . . ) is divided first into channels 1,3, 5, 7, 9, . . . traveling along path A₁ and channels 2, 4, 6, 8, 10,12, . . . traveling along path A₂ by splitter 18.

Next, channels 1, 3, 5, 7, 9, 11, . . . are divided by splitter 18, intochannels 1, 5, 9, . . . traveling along path A₁₁ and channels 3, 7, 11,. . . traveling along path A₁₂. Likewise, channels 2, 4, 6, 8, 10, 12, .. . are divided by splitter 182 into channels 2, 6, 10, . . . travelingalong path A₂, and channels 4, 8, 12, . . . traveling along path A₂₂.

Filters 32, 34, 36, and 38 then drop channels as in the WDM 24 of thepresent invention described with reference to FIG. 8. More particularly,filter 32 would drop any of channels 1, 5, 9, . . . into path C₁₁;filter 34 would drop any of channels 3, 7, 11, . . . into path C₁₂;filter 36 would drop any of channels 2, 6, 10, . . . into path C₂₁; andfilter 38 would drop any of channels 4, 8, 12, . . . into path C₂₂.

In addition, filter 32 would allow any of remaining channels 1, 5, 9, .. . to pass through into path B₁₁; filter 34 would allow any ofremaining channels 3, 7, 11, . . . to pass through into path B₁₂; filter36 would allow any of remaining channels 2, 6, 10, . . . to pass throughinto path B₂₁; and filter 38 would allow any of remaining channels 4, 8,12, . . . to pass through to path B₂₂.

Paths B₁₁ and B₁₂ are then recombined into path B₁ by splitter 26 ₁;paths B₂₁ and B₂₂ are recombined into path B₂ by splitter 26 ₂; andpaths B₁ and B₂ are recombined into path B (the output light) bysplitter 26.

Likewise, paths C₁₁ and C₁₂ are then recombined into path C₁ by splitter28 ₁; paths C₂₁ and C₂₂ are recombined into path C₂ by splitter 28 ₂;and paths C₁ and C₂ are recombined into path C (the dropped light) bysplitter 28.

In the WDM 30 of the present invention shown in FIG. 9, the channelspacing at the filters which drop the channels is four times as large asin the prior art WDM system.

Extending the example discussed with reference to the WDM 24 of thepresent invention shown in FIG. 8 to the WDM 30 of the present inventionshown in FIG. 9, if channel 3 were to be dropped, then channel 3 wouldtravel along paths A₁ then A₁₂, and be dropped by filter 34 into pathC₁₂. If channel 6 were to be dropped, then channel 6 would travel alongpaths A₂ then A₂₁, and be dropped into path C₂₁ by filter 36. Theremaining channels 1, 2, 4, 5, 7, 8, 9, . . . would continue and beoutput along path B consistent with the logic discussed herein above.

Because the separation between channels at filters 32, 34, 36, and 38 iseven greater in the WDM 30 of the present invention than the separationbetween channels being filtered in the prior art, the requirements forthe spectrum shape of the filters are less stringent and easier toachieve.

As discussed herein above, FIGS. 6 and 8 show the fundamental scheme ofthe present invention. However, isolation of the filters may have anupper limit regardless of the channel spacing. In such a case, thetunable filters may be cascaded to increase the isolation of thechannels. This scheme of the present invention is shown in FIG. 10, inwhich filters are cascaded both in the main light transmission streamand in the dropped path.

FIG. 10 shows an optical wavelength division multiplexed system (WDM) 40of the present invention with splitters and cascaded filters. In the WDM40 of the present invention, the filters are cascaded to increasechannel isolation. The WDM 40 of the present invention shown in FIG. 10is based on the WDM 24 of the present invention shown in FIG. 8. In boththe WDM 24 and the WDM 40, the channel spacing is double the channelspacing in the prior art for the light being filtered. As in the WDM 24of FIG. 8, in the WDM 40 of FIG. 10, the incoming light A (includingchannels 1, 2, 3, 4, 5, 6, 7, 8, 9, . . . ) is split by splitter 18 intolight traveling along optical path A₁ (channels 1, 3, 5, 7, 9, . . . )and light traveling along optical path A₂ (channels 2, 4, 6, 8, 10, . .. ).

Continuing in FIG. 10 with the examples discussed previously hereinabove, if channel 3 is to be dropped, then filter 20 ₁ drops channel 3into path C₁, and allows channels 1, 5, 7, 9, . . . to continue tofilter 20 ₂. Even though the channel isolation in WDM 40 (and in WDM 24)of the present invention is double the channel isolation in the priorart, there is still a small amount of leaked light (for example, 1%) ofthe dropped channel (channel 3 in the current example) that continuespast filter 20 ₁ to filter 20 ₂, with 99% of the power in the droppedchannel 3 being dropped into path C₁. Filter 202, though, allows only 1%(for example) of the 1% of the power from dropped channel 3 to continueto path B₁, with the filtered power from channel 3 continuing along pathD₁. Therefore, optical path B₁ includes in light 1, 5, 7, 9, . . . only1% of 1% (or 10⁻⁴) of the power of dropped channel 3. Therefore, thecrosstalk from dropped channel 3 is reduced by a factor of two indecibels (dB, or by the square in actual power) in path B₁.

The same would also be true for any other dropped channels, for examplechannel 6 (which would be dropped by filter 22 ₁ and 22 ₂). Paths B₁ andB₂ are then recombined by splitter 26, and output as B, with channels 1,2, 4, 5, 7, 8, 9, 10, . . . ; any further filtered power from channel 6would continue along path D₂.

From the perspective of the channel being dropped, perhaps 1% of thepower from each of the next adjacent channels (in this example, channels1 and 5) to the dropped channel 3 is also dropped into path C₁. Filter203 allows only 1% of the 1% (or 10⁻⁴) of the power from each ofchannels 1 and 5 to pass to path C₁₁, with the filtered power followingpath D₃.

Channel 6 would be dropped into path C₂ by filter 22 ₁, which path wouldalso receive perhaps 1% of the power from each of next adjacent channels4 and 8. Then, filter 22 ₃ would allow only 1% of the 1% of the powerreceived by path C₂ from each of next adjacent channels 4 and 8 to passto path C₂₁. The power from channels 4 and 8 filtered out by filter 22 ₃would then continue along path D₄.

Paths C₁₁ and C₂₁ are then recombined by splitter 28 into path C.

It is important for filters 20 ₁, 20 ₂ and 20 ₃ to be the same sincethose filters are all filtering the same channel(s). Likewise, it isimportant for filters 22 ₁, 22 ₂, and 22 ₃ to be the same since thosefilters are all filtering the same channel(s), as well. Filters 20 _(n)(where n=1, 2, or 3) are unlikely to be the same as filters 22 _(n)(also where n=1, 2, or 3).

Both the WDM 30 of the present invention and the WDM 40 of the presentinvention emphasize features of their own. The WDM 30 of the presentinvention provides effective channel spacing (4 times that of the priorart), and the WDM 40 with cascaded filters reduces crosstalk betweenchannels. The foregoing features of the WDM 30 and the WDM with cascadedfilters 40 are combined into the WDM 42 shown in FIG. 11. The WDM 42shown in FIG. 11 includes 4 paths, and all 4 paths include cascadedfilters.

The pathways and logic followed by each of the channels 1, 2, 3, 4, 5,6, 7, 8, 9, . . . included in input light A in WDM 42 shown in FIG. 11are consistent with the pathways and logic followed by each of thechannels 1, 2, 3, 4, 5, 6, 7, 8, 9, . . . included in input light A inWDM 30 of the present invention shown in FIG. 9. However, as in the caseof the WDM 40 of the present invention shown in FIG. 10, each channel inWDM 42 of the present invention shown in FIG. 11 is acted upon (filteredor allowed to pass) by two, cascaded filters. Therefore, the WDM 42 ofthe present invention isolates each channel included in input light Afrom its next adjacent channel by a distance of four channels when firstinput to a filter as in the WDM 30 of the present invention, and,further, reduces the crosstalk between dropped and remaining(non-dropped) channels by a factor of 2 in dB or by squared in terms ofpower as in the WDM 40 of the present invention.

Referring now to FIG. 11, input light A includes channels 1, 2, 3, 4, 5,6, 7, 8, 9, 10, 11, 12, . . . , and is split into paths A₁ (includingchannels 1, 3, 5, 7, 9, 11, . . . ) and A₂ (including channels 2, 4, 6,8, 10, 12, . . . ) by splitter 18. Splitter 18 ₁ then splits the lighttraveling in path A₁ into paths A₁₁ (including channels 1, 5, 9, . . . )and A₁₂ (including paths 3, 7, 11, . . . ). Likewise, splitter 18 ₂splits the light traveling in path A₂ into paths A₂₁ (including channels2, 6, 10, . . . ) and A₂₂ (including paths 4, 8, 12, . . . ).

The channels traveling in path A₁₁ are then acted upon (i.e., thechannel(s) are either dropped by the filter or allowed to pass throughthe filter) by cascaded filters 44 ₁ and 44 ₂, with channel(s) droppedby filter 44 ₁ into path C₁₁ being acted upon by filter 44 ₃. Thechannels remaining (not dropped) from input path A₁₁ are output fromfilters 44 ₁ and 44 ₂ along path B₁₁. To most effectively reducecrosstalk and increase channel isolation, filters 44 _(n) (where n=1, 2,and 3) must have the same filtering characteristics (i.e., must drop thesame channel(s)).

Filters 44 ₁ and 44 ₂ are cascaded in that the output of filter 44 ₁ isdirectly input into filter 44 ₂ (filters 44 ₁ and 44 ₂ are connected inseries) and each of the non-dropped (or remaining) channels passesthrough each of filters 44 ₁ and 44 ₂. Filters 44 ₁ and 44 ₃ arecascaded in that the output of filter 44 ₁ is directly input into filter44 ₃ (filters 44 ₁ and 44 ₃ are connected in series) and each of thedropped channels passes through each of filters 44 ₁ and 44 ₃.

Likewise, the channels traveling in path A₁₂ are acted upon by cascadedfilters 46 ₁ and 46 ₂, with channel(s) dropped by filter 46, into pathC₁₂ being acted upon by filter 46 ₃. The channels remaining (notdropped) from input path A₁₂ are output from filters 46 ₁ and 46 ₂ alongpath B₁₂. To most effectively reduce crosstalk and increase channelisolation, filters 46 _(n) (where n=1, 2, and 3) must have the samefiltering characteristics (i.e., must drop the same channel(s)).

Filters 46 ₁ and 46 ₂ are cascaded in that the output of filter 46 ₁ isdirectly input into filter 46 ₂ (filters 46 ₁ and 46 ₂ are connected inseries) and each of the non-dropped (or remaining) channels passesthrough each of filters 46 ₁ and 46 ₂. Filters 46 ₁ and 46 ₃ arecascaded in that the output of filter 46 ₁ is directly input into filter46 ₃ (filters 46 ₁ and 46 ₃ are connected in series) and each of thedropped channels passes through each of filters 46 ₁ and 46 ₃.

The light traveling in paths B₁₁ and B₁₂ is then recombined by splitter26 ₁ into path B₁.

The channels traveling in path A₂₁ are acted upon by cascaded filters 48₁ and 48 ₂, with channel(s) dropped by filter 48 ₁ into path C₂₁ beingacted upon by filter 48 ₃. The channels remaining (not dropped) frominput path A₂₁ are output from filters 48 ₁ and 48 ₂ along path B₂₁. Tomost effectively reduce crosstalk and increase channel isolation,filters 48 _(n) (where n=1, 2, and 3) must have the same filteringcharacteristics (i.e., must drop the same channel(s)).

Filters 48 ₁ and 48 ₂ are cascaded in that the output of filter 48 ₁ isdirectly input into filter 48 ₂ (filters 48 ₁ and 48 ₂ are connected inseries) and each of the non-dropped (or remaining) channels passesthrough each of filters 48 ₁ and 48 ₂. Filters 48 ₁ and 48 ₃ arecascaded in that the output of filter 48 ₁ is directly input into filter48 ₃ (filters 48 ₁ and 48 ₃ are connected in series) and each of thedropped channels passes through each of filters 48 ₁ and 48 ₃.

Likewise, the channels traveling in path A₂₂ are then acted upon bycascaded filters 50 ₁ and 50 ₂, with channel(s) dropped by filter 50 ₁into path C₂₂ being acted upon by filter 50 ₃. The channels remaining(not dropped) from input path A₂₂ are output from filters 50 ₁ and 50 ₂along path B₂₂. To most effectively reduce crosstalk and increasechannel isolation, filters 50 _(n) (where n=1, 2, and 3) must have thesame filtering characteristics (i.e., must drop the same channel(s)).

Filters 50 ₁ and 50 ₂ are cascaded in that the output of filter 50 ₁ isdirectly input into filter 50 ₂ (filters 50 ₁ and 50 ₂ are connected inseries) and each of the non-dropped (or remaining) channels passesthrough each of filters 50 ₁ and 50 ₂. Filters 50 ₁ and 50 ₃ arecascaded in that the output of filter 50 ₁ is directly input into filter50 ₃ (filters 50 ₁ and 50 ₃ are connected in series) and each of thedropped channels passes through each of filters 50 ₁ and 50 ₃.

The light traveling in paths B₂₁ and B₂₂ is then recombined by splitter26 ₂ into path B₂.

The light traveling in paths B₁ and B₂ is recombined by splitter 26 intopath B.

Likewise, the dropped light traveling in paths C₁₁ and C₁₂ is recombinedby splitter 28 ₁ into path C₁; the dropped light traveling in paths C₂₁and C₂₂ is recombined by splitter 28 ₂ into path C₂; and the droppedlight traveling in paths C₁ and C₁₂ is recombined by splitter 28 intopath C.

If channel 3, for example, were selected as the dropped channel, thenchannel 3 would travel along path A₁, then path A₂₁, then be droppedinto path C₂₁ by filter 48 ₁. Perhaps, though, 1% of the power ofchannel 3 continues along with remaining channels 7, 11, . . . . Thenfilter 48 ₂ would allow only 1% of the 1% (or 10⁻⁴) of the power fromchannel 3 to continue along path B₂₁ at the output of filter 48 ₂.Likewise, filter 48 ₃ would reduce the amount of power included in anynon-dropped channels erroneously traveling in path C₂₁ by an additional(for example) 10⁻² (which power was already reduced by 10⁻² by filter 48₁), so that only 10⁻⁴ of the power of the non-dropped channels input tofilter 48 ₁ is output by filter 48 ₃.

In the present invention, the splitter is an important device. Onestructure which can be used as a splitter to split the even channels andthe odd channels is a Mach-Zehnder interferometer with asymmetric armlengths, as shown in FIG. 12. Mach-Zehnder interferometers withasymmetric arm lengths, generally, are well-known in the art, and thedifference in the arm lengths of the Mach-Zehnder interferometer shownin FIG. 12 determines the channel spacing. In addition, 50/50 couplersare well-known in the art.

As shown in FIG. 12, input light 1 (including channels 1, 2, 3, 4, . . .) is input to the Mach-Zehnder interferometer 52 of FIG. 12. The inputlight 1 is then directed by conventional 50/50 coupler 54 along paths Dand E.

Along path D, the input light 1 is reflected by mirrors 56 and 58 to50/50 coupler 60. Along path E, the input light 1 travels directly to50/50 coupler 60. Then, 50/50 coupler recombines the light traveling inpaths D and E to produce output light 2 (including only channels 1, 3, .. . ) and output light 3 (including only channels 2, 4, . . . ). Therelative phase of the light at 50/50 coupler 60 determines the channelsoutput along output path 2 and output path 3, using the formulas of:relative phase, θ=((2*π*n*f)/c)*(D _(D) −D _(E))  (1)and output power=½+½ cos θ  (2)

where D_(D) is the distance along path D, D_(E) D_(E) is the distancealong path E, n is the refractive index (1 for air), f is the frequencyof the input light 1, c is the speed of light, and π is 3.14 . . .

The above-mentioned formulas (1) and (2) also determine the period ofthe output light, and, by changing (D_(D)−D_(E)), the period of theoutput light with respect to the frequency f can be changed to matchtwice the channel spacing.

Waveguides, generally, are known in the art. The splitter could,alternatively, be constructed as a waveguide device 60 with the shapeshown in FIG. 13 and having the same mathematics as the Mach-Zehnderinterferometer 52 shown in FIG. 12. The waveguide device 60 shown inFIG. 13 is an asymmetric waveguide or fiber scheme in which input light1 is input to 50/50 coupler 62, travels along paths F and G of differentlengths, and is recombined by 50/50 coupler 64 to produce outputs 2 and3, consistent with the above-mentioned discussion with reference to FIG.12.

One disadvantage of the Mach-Zehnder type of interferometer or splitteris that the filtering spectrum is not sharp. Typical spectra in twooutputs of a Mach-Zehnder interferometer are shown by the solid line “2”and the dotted line “3”, respectively, in FIG. 14. The spectrum shown bythe solid line “2” in FIG. 14 is of transmissivity (dB or log(power) vs.frequency); the channels with the odd numbers should be transmitted(such as in output “2” of FIGS. 12 and 13) and the channels with theeven numbers should be rejected in this output (such as in output “3” ofFIGS. 12 and 13). But the rejection bandwidth (the bandwidth in thespectrum in which light power is rejected) is narrow and the light fromthe even numbered channels will leak into the output. Although theoutput shown in FIG. 14 is periodic, the output is not rectangular,meaning that crosstalk between channels is likely to be present. Arectangular output, having a broad bottom, is needed to ensure thatcrosstalk is avoided.

A desirable spectrum is shown in FIG. 15, in which the shape of thespectrum is more rectangular, and, therefore, approaches more closelythe rectangular shape of an ideal spectrum.

FIG. 16A shows an ideal splitter 66, having input light 1 with channels1, 2, 3, 4, 5, 6, 7, 8, 9, . . . . Splitter 66 splits the input lightinto paths 2 (having channels 1, 3, 5, 7, 9, . . . ) and 3 (havingchannels 2, 4, 6, 8, . . . ). FIG. 16B shows the spectrum of thesplitter in path 2, and FIG. 16C shows the spectrum of the splitter inpath 3. As shown in FIGS. 16B and 16C, since the spectrum isrectangular, even if the position of the wavelength within each channelfluctuates within the channel, the log(power) or the dB always goes lowin (for example) channel 4 in FIG. 16B (as indicated by Δ), or tomaximum in channel 3 of FIG. 16B (as indicated by X). Likewise, thelog(power) or dB always goes low for channel 3 in FIG. 16C (as indicatedby Δ), and to maximum for channel 4 in FIG. 16C (as indicated by X).

An ideal spectrum for a splitter, including splitter 66, would berectangular and periodic. Periodicity is important because there aremany WDM channels, which need to repeat 20–80, even 100, times. Theabove-mentioned asymmetric Mach-Zehnder interferometer providesperiodicity, but not necessary a rectangular shape of the spectrum.

A scheme to realize a more ideal spectrum is shown in FIG. 17. In thesplitter 68 shown in FIG. 17, the input light 1 to the splitter 68 isdivided equally into two arms A and B by 50/50 coupler 70. Arms A and Brespectively include phase elements 72 and 74. The light A₁ and B₁ arerecombined by 50/50 coupler 76 after the phase elements 72 and 74. Thephase elements 72 and 74 add an optical phase onto the traveling light Aand B, which changes periodically as shown in FIG. 18.

The output power from splitter 68 depends upon the relative phase θbetween the two arms A₁ and B₁ in the same manner as in formula (2). Inthe asymmetric Mach-Zehnder interferometer previously described, therelative phase is generated by the path lengths. However, the splitter68 shown in FIG. 17 generates the relative phase in other ways, throughthe inclusion of the phase elements 72 and 74. Therefore, the lengths ofboth pathways (beginning with arms A and B) in the splitter 68 can bethe same, but the splitter 68 is not required to be symmetric.

The phase elements 72 and 74 each add an optical phase which isdetermined by the frequency, as explained with reference to FIG. 18.

The solid line shown in FIG. 18 indicates the added phase in one arm(path A₁) and the dashed line indicates the added phase in the other arm(path B₁) of splitter 68 of FIG. 17. The output light goes to eitheroutput 2 or output 3 depending the relative phase, which is thedifference (P_(D)) between the solid line A₁ and the dashed line B₁ inFIG. 18. The phase difference P_(D) can range between −δ and π+δ, whereδ is a small positive angle between 0 and 0.1 radians. The phase curveshown in FIG. 18 need not be sinusoidal, but must be periodic.

The two arms of the light paths A (including A₁) and B (including B₁) ofFIG. 17 can be either light paths in different spaces or orthogonalpolarizations of the light in the same space. The splitter 68 of FIG.17, therefore, provides a better spectrum, approaching the idealspectrum shown in FIG. 15, over that of the prior art.

The following three schemes are example implementations of the phaseelements 72 and 74 of FIG. 17. The three schemes, shown in FIGS. 19, 20,and 21, are all well known in the art, are mathematically equivalent toeach other, and operate under the same principle to add the opticalphase shown in FIG. 18. More particularly, in the schemes shown in FIGS.19–21, a portion of the input light travels through a longer path and isthen recombined with the input light.

In the phase element 78 of FIG. 19, the majority of input light 1 (A orB corresponding to FIG. 17) is reflected by partial mirror 80 to outputpath A₁ (for phase element 72 of FIG. 17) or output path B₁ (for phaseelement 74 of FIG. 17). However, a portion of the input light 1 isdivided out of the input light by partial mirror 80 and the dividedportion of the input light travels a longer light path P, reflected bymirrors 82, 84, and 86, then partially recombines at partial mirror 80with the input light 1 to affect the optical phase of either outputlight A₁ or B₁. A₁ and B₁, then, are the combination of the output powerof light 1 and the part of the input light 1 traveling the longer path Pthrough one or more round trips. The optical phase of output power of A₁and B₁, then, depends upon whether the portion of the input light 1traveling the longer path P is in-phase, out-of-phase, opposite-phase,etc., with the input light 1 being reflected directly along path A₁ orB₁.

Similar principles apply to the phase element 88 shown in FIG. 20, whichuses a waveguide coupling with a loop 92, and to the phase element 94shown in FIG. 21, which uses a partial mirror 96 and a mirror 98, asshown to accomplish the functions of the phase element 78 shown in FIG.19.

The phase element 88 shown in FIG. 20 is a waveguide device comprising amain waveguide 90 and a waveguide loop 92 partially coupled to the mainwaveguide 90 at a position C on the waveguide loop 92.

The phase element 94 shown in FIG. 21 includes a partial mirror 96allowing the input light A or B to pass through and a mirror 98 off ofwhich the input light A or B is reflected, wherein the input lighttravels a path P of being reflected between the partial mirror 96 andthe mirror 98 until being output from the phase element 94 through thepartial mirror 96 along path A₁ or B₁.

FIG. 22 is a flowchart of the methods of the optical wavelength divisionmultiplexed (WDM) systems of the present invention. As shown in step S1of FIG. 22, input light is received by the previously-described WDMsystems 24, 30, 40, or 42 of the present invention. Then, in step S2,the channels carried in the input light are divided into n paths, wheren>1, and no two consecutive channels are allocated to the same path.More particularly, the channels carried in the input light are dividedinto 2 paths in the WDM 24 of the present invention shown in FIG. 8 andin the WDM 40 of the present invention shown in FIG. 10, and into 4paths in the WDM 30 of the present invention shown in FIG. 9 and in theWDM 42 of the present invention shown in FIG. 11. Alternatively, aspreviously described, the channels carried in the input light could bedivided into 3 paths, etc.

Next, in step S3, the WDM of the present invention drops the channel(s)carrying the light to be dropped into dropped paths, using thepreviously-described cascaded or non-cascaded filters. In step S4, thefilters allow the channels which remain and are not dropped to betransmitted within the WDM of the present invention in transmissionpaths.

The dropped paths are recombined into a dropped light system output path(for example, path C in FIGS. 8, 9, 10, and 11) by the WDM of thepresent invention, in step S5. In addition, the remaining light isrecombined into a transmitted light system output path (for example,path B in FIGS. 8, 9, 10, and 11), in step S6.

Then, the WDM of the present invention outputs the dropped light systemoutput path and the transmitted light system output path.

The many features and advantages of the invention are apparent from thedetailed specification and, thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirit and scope of the invention. Further, sincenumerous modifications and changes will readily occur to those skilledin the art, it is not desired to limit the invention to the exactconstruction and operation illustrated and described, and accordinglyall suitable modifications and equivalents may be resorted to, fallingwithin the scope of the invention.

1. A method of splitting input light having a first channel and a secondchannel, comprising: dividing the input light equally into two arms,adding optical phases to the divided input lights, which changeperiodically with a respective optical frequency domain, and splittingthe first channel and the second channel based upon the relative phasebetween the divided input light.
 2. An optical wavelength divisionmultiplexed system, comprising: a splitter splitting an input light intoa first light of a first path and a second light of a second path; afirst phase element adding phase to the first light that changesperiodically on optical frequency domain, the first phase elementarranging into the first path; a second phase element adding phase tothe second light that changes periodically on optical frequency domain,the second phase element arranging into the second path; and a couplercombining the first path and the second path, the coupler dividing aneven number channel path and an odd number channel path.
 3. A method ofsplitting a channel carried by input light received by an opticalwavelength division multiplexed system, comprising: splitting thechannel into one path and next higher and lower-numbered channels fromthe channel into another path, said splitting including: dividing theinput light equally into two arms, adding an optical phase to thedivided input light which changes periodically, and splitting thechannel and the next higher and lower-numbered channels based upon therelative phase between the divided input light.
 4. The method accordingto claim 3, wherein the channel is odd-numbered and the next higher andlower channels are even-numbered.
 5. The method according to claim 3,wherein the optical phase of the divided input light changesperiodically with a frequency domain.
 6. A method of isolating channelscarried by input light, comprising: splitting the channels into separatepaths, one paths carrying odd-numbered channels and another of the pathscarrying even numbered channels, said splitting including: dividing theinput light equally into two arms, adding an optical phase to thedivided input light which changes periodically, splitting the channelsand one of the paths and another of the paths based upon the relativephase between the divided input light.
 7. The method according to claim6, wherein the optical phase of the divided input light changesperiodically with a frequency domain.
 8. The method of splitting inputlight according to claim 1, wherein the optical phases have a phasedifference approximately Π on optical frequency domain.
 9. The opticalwavelength division multiplexing system according to claim 2, whereinthe first phase element's phase and the second phase element's phasehave a phase difference approximately Π on frequency domain.